Search for OH in the disk around HD163296

Coordinator: H.-U. Kaeufl (ESO-Garching)

Co-Is: B. Bonev, A. Mandell, M. Mumma, G. Villanueva


We propose to observe the HAe star HD163296, known to have a circumstellar disks. We will search for emission from ro-vibrational transitions of OH, identified but spectrally unresolved in preliminary observations of several similar HA stars with disks. CRIRES will offer improved spectral resolution, allowing us to resolve the spectral line shape and disentangle the source of this emission.

Allocated Time: 2 hours

Targets list

NameRA(2000)DEC(2000)Range (nm)Wavelength IDF_nu or F_lineDIT(s),NDIT
HD 16329617 56 21.3-21 57 19.52933-300319/0/n6.3e10^-17 W/m^230, 120
HD 16395517 59 47.6-23 48 58.12933-300319/0/n3.0e10^-16 W/m^215, 60

Project description/scientific objective:

Observations of spectral features in the near infrared (NIR) spectral region offer a window into the hot inner and surface regions of circumstellar disks. CRIRES represents an exceptional improvement in spectral resolution at NIR wavelengths, allowing improved analysis of line shape due to thermal and velocity broadening. We propose to utilize this advantage to analyze a new constituent of circumstellar disks, the OH molecule. CO ro-vibrational emission at 4.7mum has been detected in both HAeBe and T Tauri stars (Brittain et al. 2003, Blake & Boogert 2004, Dent et al. 2005), as well as hot H2O (Carr et al. 2004, Thi & Bik 2005), organics (Lahuis et al. 2006, Habart et al. 2006) and warm H2 emission in various environments (Bary et al. 2003, Richter et al. 2005). Modeling of these disks suggests that there are additional molecular species that may be abundant and detectable. Under the intense UV radiation field from young stars, the dominant molecular disk constituents of H2 and CO are easily dissociated into a range of atomic and molecular by-products, which in turn participate in a complex combination of exothermic and endothermic reactions (Glassgold et al. 2004, Thi & Bik 2005, Kamp et al. 2006). Of particular importance are the ions of OH+, HCO+ and H3+, as well as neutral OH and O2; these molecules compose the intermediate stages of most of the important reactions at work in high-temperature low-density environments. We have taken preliminary observations of several HAeBe stars with low-inclination disks in the L band to search for OH ro-vibrational emission lines, and initial results are positive. Though most sampled low-J OH transitions are heavily attenuated by telluric absorbtion, we have detected the (1-0) band P3.5 1+ and 1- doublet at 3407.5 cm-1 in both AB Aurigae and MWC 758. The features have the same velocity shifts as the nearby high-J HI and H2 lines, and may therefore be associated with accretion activity (Herczeg et al. 2005). However, the linewidths for the OH lines appear unresolved at R ~ 25,000, giving an upper limit for the line width FWHM of 20 km/s. Since the HI and H2 lines have FWHM of ~300 km/s, the origin of these features requires further consideration. We propose to observe HD163296, another bright HAe star with a low-inclination disk, to search for these emission lines. CRIRES will give us improved resolution in order to obtain the velocity profile of any detected emission, and therefore assertion the radial location and/or thermal environment of the material producing this emission. High spectral resolving power is critical for this analysis, and a detection and resolution of the line shape will demonstrate the utility of this instrument for the exploration of planet-forming regions.

We have developed reduction techniques that combine the removal of both a synthetic terrestrial atmospheric model and a comparison star. The use of a synthetic spectrum allows us to compensate for variations in airmass and water vapor between the science star and the comparison star, and the subtraction of the comparison star compensates for telescope systematics and unknown atmospheric constituents. We have achieved photon noise-limited residuals with S/N per pixel of ~2000 with data from similar stellar targets on other NIR spectrographs.

We will compare the results to models of OH fluorescence and prompt emission developed for work on comets. We have observed vibrational prompt emission from OH in several comets via high resolution infrared spectroscopy (Mumma et al. 2001, Bonev et al. 2004, Bonev et al. 2006). The principal excitation mechanism for this emission is a single-step photolysis of H2O, terminating in OH fragments that are both vibrationally and rotationally excited. The resulting infrared emission traces spatially the distribution of the parent molecule (H2O). In addition to prompt, fluorescent ro-vibrational emission from OH in comets can originate from levels of low (J ~4.5) rotational excitation, populated via direct infrared solar pumping or via UV fluorescence through the A2\u03a3 states followed by cascade into X2\u03a0 (Despois et al. 1981, Schleicher & A\u2019Hearn 1988). Secure detections (via multiple lines) of OH in circumstellar disks will provide information for the likely excitation mechanism (prompt or fluorescent emission) for OH, which in turn reflects the conditions in the corresponding part of the disk. Similarly to comets, in case of prompt emission, the infrared quanta from OH could be a direct signature of H2O photolysis, occurring in the same spatial region from the disk where the emission from OH originates.

Observing strategy

The science star is HD163296, the first star in the list. The spectrum of the comparison star, HD163955, will be subtracted from that of the science star to remove telescope and atmospheric systematics. Therefore it is important that the minimum changes in the telescope set-up be made between calibration images (flats and darks) and observations of both stars. If possible, please take flats and darks immediately before or after the observations.

Previous observations suggest a S/N of 1000 will be required to detect the OH features in the science target; the total estimated exposure time is based on the CRIRES calculator and manual, but may need to be adjusted.

Please observe both stars as close in time as possible, without changing the wavelength settings, in order to minimize systematics. Both stars are bright enough for self-referencing of the AO.